Mystery Wire (AP) — At the end of stars’ lives, they collapse under their own weight and burst into a supernova. The heaviest leave behind black holes, lighter ones birth neutron stars.
“Slightly lighter stars produce neutron stars, which are actually a bit more like tiny, ultra-dense planets. And they’re so dense that a teaspoon of neutron star material would weigh as much as Mount Everest,” explains Anna Watts, a professor of astrophysics at the University of Amsterdam.
Using NICER (Neutron star Interior Composition Explorer), an x-ray telescope perched on the International Space Station, astronomers have for the first time measured the size of the heaviest known neutron star.
It’s called PSR J0740+6620 – J0740, for short – and is about 3,600 light years away from Earth. It was discovered in 2019.
It’s more than twice our own Sun’s mass, but measures just 16 miles across, not much bigger than Manhattan Island.
Their studies are based on about a year and a half of telescope observations.
“What NICER is doing is looking for x-rays emitted from the super-hot surfaces of neutron stars. We use that to measure how big the stars are, and that tells us essentially what they’re made of,” explains Watts, group lead for one of the teams.
“So, what we’ve now done is we’ve used NICER to measure the mass of the heaviest known neutron star. So, indeed, it’s two times the mass of the sun, but it’s only 16 miles across.”
Watts and her colleagues found that J0740 was actually not as small as they might have expected, thus challenging theories concerning the dense matter at the stars’ hearts.
It’s “less squashy” than some physicists predicted, she says.
“The various different ideas that people have about the kind of particles that exist in neutron stars indeed sets how squashy the star is. Some of the models that existed previously said actually, they could be pretty squashy. So, they could be pretty small, even when they contain a huge amount of matter,” explains Watts.
“When we measured the star, we actually found it’s not as small as we might have expected. It’s more of a kind of mid-size star. So, that tells us that some of these more squashy models essentially don’t work.”
The team’s findings are also helping inform theories on what kind of cosmic oddities eventually become black holes.
“One of the things it tells us potentially is what is the maximum mass that a neutron star can have before it collapses into a black hole,” says Watts.
“We have colleagues also doing analysis with gravitational waves, trying to come at this from a different direction and finding smaller and smaller black holes, and we’re trying to work out where the upper limit of neutron stars is. And at some point, there’s a line, beyond which something has to become a black hole.”
The team presented their findings at a virtual meeting of the American Physical Society on April 17.
Papers describing their findings are currently undergoing scientific review.